A method for determining left ventricular heat dissipation in intact dogs has been developed. In this method, a known quantity of heat is injected into the myocardium via the left coronary arterial inflow and the percentage of this "excess heat" recovered in the coronary sinus is measured. It is assumed that this percentage figure is the same for the fraction of the total left ventricular heat which is dissipated via the coronary sinus outflow. Knowing this percentage figure, the total left ventricular heat can be derived from measuring coronary sinus heat dissipation. From simultaneous measurements of heat dissipation and work rate, the total energy liberation by the left ventricle is obtained. This direct measurement of total energy liberation will now be evaluated against the direct measurement from metabolic data. In future work the mechanical efficiency of the left ventricle will be studied as work/work plus heat. The magnitude and variability of left ventricular efficiency will be evaluated under the effects of inotropic interventions, infarction, hypertension and hypertrophy. Also, this investigation will be able to document whether or not some of the known cardiovascular control mechanisms involve, among other parameters, changes in left ventricular efficiency.